Both ministerial and Whitehall officials stressed the similarities between the London attacks and the train bombings in Madrid last year which killed 191 people.

The man believed to be the Madrid mastermind, Mustafa Setmariam Naser, a 47-year-old Syrian with dual Spanish nationality, is said to be “one of many suspects” being hunted by the police and MI5. They also want to question Zeeshan Hyder Siddiqui, 25, a British national allegedly trained to make bombs in an al-Qaeda camp who was arrested in Pakistan in May.

Their search is being hampered because those who planted the bombs are believed to be “cleanskins”, young operatives with no known terrorist links.

The secret services, The Sunday Telegraph understands, picked up no intelligence that could have alerted them to the possibility of attacks on London. There was none of the “chatter” – information picked up by electronic surveillance – or suspicious financial transactions that have helped to thwart other planned attacks.

“We are convinced it is not a British-based cell,” a senior Government source said. There was a strong possibility that the bombers came from Iraq and spent time in mainland Europe before entering Britain recently.

Two theories have emerged. The first is that the terrorist cell, which calls itself Secret Organisation of the al-Qaeda Jihad in Europe, is composed of British Muslims, unknown to the intelligence services, who have become radicalised by the wars in Afghanistan and Iraq and have signed up to the al-Qaeda philosophy that the Western way of life must be destroyed.

On this hypothesis, the group is small in number, less than 20 but more than 12, and composed of educated men and women in their late twenties and early thirties.

It is highly likely that they are being lead by a seasoned terrorist, possibly from outside Britain, who is capable of obtaining commercial or military explosives and making small but lethal bombs, and has a record of past success in co-ordinating attacks.

The second and more likely theory, The Sunday Telegraph has learned, which has been presented to ministers during their daily Cobra anti-terror committee briefings, is that the terrorists are from abroad – either Europe or North Africa – and entered Britain during the past six months with false passports.

Steve Emerson reported on the North Africa theory on MSNBC at at The Counterterrorism Blog here.